Align ddp and ddp-spawn strategies in setting up the environment

[awaelchli]

What does this PR do?

Aligns the regular ddp strategy and the ddp spawn strategy in setting up the environment (through the method setup_environement which is called in the Trainer._run).

Before:

DDP: Setup environment in setup_environment method/hook.
DDPSpawn: At spawn time, in worker_setup function (slightly earlier than setup_environment method/hook

Now:

Both DDP and DDPSpawn set the same variables in setup_environment method.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @kaushikb11 @akihironitta

[ananthsub]

I believe this will be a prerequisite for further consolidation efforts like #11021 and #11020 by moving up the setup_environment of DDP and TPU spawn

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/generated/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

[tchaton]

Hey @awaelchli. What is the status on this PR ?

[awaelchli]

The status is that I'm currently working on this and once #11163 is merged this will pretty much be ready.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/generated/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/generated/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

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[Borda]

@awaelchli mind checking the conflicts... 🦦

[carmocca]

🎉

[awaelchli]

🥳